RETINAL CHANGES INVOLVED IN VISION 625 



inner molecular layer, the so-called ' amacrine ' cells. These cells 

 have been imagined to serve as a means of connection or association 

 between different parts of the retina, and may be taken as analogous 

 to the association-cells found in the cerebral cortex. The analogy of 

 the retina with a lobe of the brain is illustrated by the fact that, in 

 addition to the fibres originating in the retina and passing towards the 

 brain, a considerable number of fibres pass from the central nervous 

 system into the retina, where they end chiefly in the two molecular 

 layers. These may have as one of their functions the correlation 



i\\i 



FIG. 285. Section through half the fovea centralis. (SCHAFEB 

 and GOLDING BIRD.) 



of processes occurring in the retinae of the two eyes and may be 

 associated with phenomena such as those of binocular contrast, which 

 we shall have to study later on. 



Important differences are found in the structure of the retina 

 in its different parts. At the point of entrance of the optic nerve 

 the optic disc the only elements present are the nerve fibres, which 

 diverge from this point over the whole inner surface of the retina. 

 A short distance externally to the optic disc is found the macula lutea 

 with a small depression in the middle, the central spot on fovea centralis 

 (Fig. 285). When we fix our gaze on any object the visual axes are 

 so directed that the image of the object falls on the fovea centralis. 

 At this spot the retina is thinned by the gradual disappearance of all 

 its layers except the outermost. The outermost layer is moreover 

 distinguished by the fact that the rods have disappeared and that 



40 



